The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the reconstruction of images from cardiac gated magnetic resonance acquisitions.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1, is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a “view” and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, or k-space samples, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
The most prevalent method for acquiring an NMR data set from which an image can be reconstructed is referred to as the “Fourier transform” imaging technique or “spin-warp” technique. This technique is discussed in an article entitled “Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging”, by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, p. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (Gy) in the sequence of views that are acquired during the scan. In a three-dimensional implementation (3DFT) a third gradient (Gz) is applied before each signal readout to phase encode along the third axis. The magnitude of this second phase encoding gradient pulse Gz is also stepped through values during the scan. These 2DFT and 3DFT methods sample k-space in a rectilinear pattern as shown in FIG. 2 and the k-space samples lie on a Cartesian grid.
Magnetic resonance angiography (MRA) uses the magnetic resonance phenomenon to produce images of the human vasculature and heart. To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick with this contrast enhanced (CE) MRA method is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CEMRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds.
The acquisition of MRA data is timed such that the central region of k-space is acquired as the bolus of contrast agent arrives in the arteries of interest. The ability to time the arrival of contrast varies considerably and it is helpful in many applications to acquire a series of MRA images in a dynamic study which depicts the separate enhancement of arteries and veins. A temporal series of images is also useful for observing delayed vessel filling patterns caused by disease. This requirement has been partially addressed by acquiring a series of time resolved images using a 3D “Fourier” acquisition as described by Korosec F., Frayne R, Grist T., Mistretta C., “Time-Resolved Contrast-Enhanced 3D MR Angiography”, Magn. Reson. Med. 1996; 36:345-351 and in U.S. Pat. No. 5,713,358.
More recently projection reconstruction methods have been used for acquiring time-resolved MRA data as disclosed in U.S. Pat. No. 6,487,435. Projection reconstruction methods, sometimes referred to as “radial” acquisitions, have been known since the inception of magnetic resonance imaging. Rather than sampling k-space in a rectilinear scan pattern as is done in Fourier imaging and shown in FIG. 2, projection reconstruction methods acquire a series of views that sample radial lines extending outward from the center of k-space as shown in FIG. 3. The number of views needed to sample k-space determines the length of the scan and if an insufficient number of views are acquired, streak artifacts are produced in the reconstructed image. The technique disclosed in U.S. Pat. No. 6,487,435 reduces such streaking by acquiring successive undersampled images with interleaved views and sharing peripheral k-space data between successive images.
There are two methods used to reconstruct images from an acquired set of k-space projection views as described, for example, in U.S. Pat. No. 6,710,686. The most common method is to regrid the k-space samples from their locations on the radial sampling trajectories to a Cartesian grid. The image is then reconstructed by performing a 2D or 3D Fourier transformation of the regridded k-space samples. The second method for reconstructing an image is to transform the radial k-space projection views to Radon space by Fourier transforming each projection view. An image is reconstructed from these signal projections by filtering and backprojecting them into the field of view (FOV). As is well known in the art, if the acquired signal projections are insufficient in number to satisfy the Nyquist sampling theorem, streak artifacts are produced in the reconstructed image.
The standard backprojection method is illustrated in FIG. 4. Each acquired signal projection profile 10 is backprojected onto the field of view 12 by projecting each signal sample 14 in the profile 10 through the FOV 12 along the projection path as indicted by arrows 16. In backprojecting each signal sample 14 in the FOV 12 we have no a priori knowledge of the subject and the assumption is made that the NMR signals in the FOV 12 are homogeneous and that the signal sample 14 should be distributed equally in each pixel through which the projection path passes. For example, a projection path 18 is illustrated in FIG. 4 for a single signal sample 14 in one signal projection profile 10 as it passes through N pixels in the FOV 12. The signal value (P) of this signal sample 14 is divided up equally between these N pixels:Sn=(P×1)/N  (1)where: Sn is the NMR signal value distributed to the nth pixel in a projection path having N pixels.
Clearly, the assumption that the NMR signal in the FOV 12 is homogeneous is not correct. However, as is well known in the art, if certain filtering corrections are made to each signal profile 10 and a sufficient number of filtered profiles are acquired at a corresponding number of projection angles, the errors caused by this faulty assumption are minimized and image artifacts are suppressed. In a typical, filtered backprojection method of image reconstruction, 400 projections are required for a 256×256 pixel 2D image and 203,000 projections are required for a 256×256×256 voxel 3D image. If the method described in the above-cited U.S. Pat. No. 6,487,435 is employed, the number of projection views needed for these same images can be reduced to 100 (2D) and 2000 (3D).
When imaging certain arteries, such as coronary arteries, the motion of the beating heart becomes an issue. To reduce motion artifacts in MRI or MRA images it is common practice to cardiac gate the acquisition of views using an ECG signal indicative of cardiac phase. As described, for example, in U.S. Pat. No. 5,329,925 a group, or segment, of views are acquired at each of one or more cardiac phases during each cardiac cycle. For example, 8 different views may be acquired at a particular cardiac phase and after 16 heart beats a total of 8×16=128 different views are acquired from which an image may be reconstructed. Since a single breath-hold is typically 16-20 heartbeats it is highly desirable to acquire all the data within breath-hold in order to avoid artifacts due to respiratory motion.
While a decent single-slice, 2D image may be acquired at one or more cardiac phases during a single breath-hold using projection reconstruction methods and view sharing, prior methods are not fast enough to acquire a 3D image or multiple 2D slices at each cardiac phase. Such images are necessary when the subject of the examination does not lie in a single 2D plane (e.g., coronary arteries) and either a multi-slice or 3D image acquisition is needed.